Repeated administration of long-acting hallucinogens rapidly elicits tolerance to their psychological effects. N,N-Dimethyltryptamine (DMT) is a very short acting endogenous hallucinogen & drug of abuse, tolerance to which has never been demonstrated in humans. We will attempt to develop tolerance in a pilot group of three subjects by gradually increasing doses and frequency of administration of 4 consecutive doses, and then studying 12 experienced hallucinogen users using repeated placebo and repeated DMT on 2 separate days.